Compared to other treatment options, including brachytherapy, chemotherapy, and prostatectomy, external beam radiation therapy (ERBT) has been documented as an effective treatment modality for prostate cancer in all stages [1], especially with advancements in conformal radiation treatment (3-dimensional conformal radiation therapy (3DCRT); proton therapy (PT); intensity-modulated radiotherapy (IMRT)), which allow for increased normal tissue sparing while, simultaneously, escalating the radiation dose [2].
Despite a well-documented dose-outcome relationship regarding local tumor control and biochemical failure post-treatment for doses as high as 86.4 Gy [3], the prostate cancer ERBT remains limited by the ability to deliver a sufficient dose of radiation to the tumor without toxicity to bladder and rectum. However, to ensure dosimetric coverage of the prostate (clinical target volume or CTV), planning target volume (PTV) margins need to account for technical uncertainties, as well as prostate motion and specific circumstances, such as the stereotactic or hypofractionated prostate. The higher the CTV-to-PTV margin, the higher the risk of normal tissue irradiation.
According to several recent studies, a rectal balloon for radiation therapy, with reproducible intra-pelvic position, significantly limits inter- and intrafraction motion of the prostate, allowing the therapist to potentially reduce the PTV margin [3].
Study of the real-time intrafraction motion (Both et al.) showed that the use of a daily-ERB consistently stabilized prostate. Within 6-minute treatment time (over 787 tracking sessions in 24 patients), three-dimensional (3D) displacements of >6 mm were negligible, displacements of >5 mm were independent of treatment time, and displacements of >3 mm occurred in less than 5% of treatment time. The results suggest that an internal margin (IM) of as low as 3 mm can cover 95% of treatment time under 6 minutes while 5 mm IM can sufficiently cover 95% of treatment time over 6 minutes. Lateral (L) displacements were negligible while cranial-caudal (CC) and anterior-posterior (AP) were comparable. The study of the intrafraction motion between the groups of ERB and non-ERB patients (30 and 29 patients, 1,008 and 1,061 tracking sessions respectively) (Wang et al.) confirmed the findings, suggesting that 3D IM can be reduced by 40% with the use of daily-ERB during EBRT [2].
In addition to lower PTV (dosimetric effects), which allows further dose escalation and improved clinical outcomes, a rectal balloon for radiation therapy increases the sparing of the anterior rectal wall and reduces the surrounding tissues volume receiving high-dose radiation through rectal wall distension [4].
[1] American Cancer Society, Inc. Initial Treatment of Prostate Cancer, by Stage. March 2016. [Online]. Available: https://www.cancer.org/cancer/prostate-cancer/treating/by-stage.html.
[2] Both S, Deville C, Bui V, Wang KK-H, Vapiwala N. Emerging evidence for the role of an endorectal balloon in prostate radiation therapy. Translational Cancer Research. Oct. 2012; 1(3): 227-235.
[3] Cho JH, Lee C-G, Kang DR, Kim J, Lee S, Suh C-O, Seong J, Suh YG, Lee I, Kim GE. Positional Reproducibility and Effects of a Rectal Balloon in Prostate Cancer Radiotherapy. The Journal of Korean Medical Science. Oct. 2009; 24(5): 894–903.
[4] Teh BS, McGary JE, Dong L, Mai WY, Carpenter LS, Lu HH, Chiu JK, Woo SY, Grant WH, Butler EB. The use of rectal balloon during the delivery of intensity modulated radiotherapy (IMRT) for prostate cancer: more than just a prostate gland immobilization device? Cancer journal. Nov.-Dec. 2002; 8(6): 476-483.
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